The invention concerns a method for generating MR (magnetic resonance) images of a moving object having a repeating motion pattern in comparable motion states, wherein a fully encoded set of MR data for producing an MR image is provided from a plurality of successive individual MR measurements for at least one motion state, the time period between the individual MR measurements being shorter than a repetition rate of the motion pattern, and wherein at least one data point from each individual MR measurement is used as an indicator for the comparability of several motion states.
Methods of this type for imaging moving objects are disclosed in the Journal of Magnetic Resonance in Medicine, MRM 13 (2005).
High-resolution MR images of moving objects must be recorded, in particular, for clinical applications. These are mainly recordings of the heart or abdominal region, wherein the object to be imaged moves relative to the magnetic field generated by the MR apparatus due to heart beat and breathing. The detection of a 2- or 3-dimensional image requires repeated application of imaging pulse sequences with different phase encoding gradients and reconstruction of the image. Due to a continuous motion pattern of the object which usually involves a plurality of motion states, the individual MR signals are recorded in different motion states. This can produce artefacts, such as ghostings, distortions and deterioration of the resolution in the reconstructed image.
This problem is conventionally solved by triggering the times of the individual measurements to comparable motion states of the object using external sensors. However, this requires more time, in particular, when several individual motions of different frequencies overlap. With this method, a particular flank of the trigger signal, which characterizes a certain motion state of the object, must always be waited for. The spin system could therefore be in different relaxation states which, in turn, produces different signal strengths.
Another approach which avoids external trigger signals is the “back projection” method. An indicator is recorded with full rephasing of the spins without using a phase encoding gradient, such that the echo maximum is recorded as the indicator. Since the motion pattern of the object can result in amplitude fluctuations of the indicator signal, a certain size of the amplitude can be associated with a corresponding layer of the object to be imaged. However, this method cannot be applied to 2D-FT methods due to spin dephasing. Another disadvantage is the fact that only one point, i.e. the echo maximum, serves as the indicator. This can cause inaccuracies in the determination of the motion state.
U.S. Pat. No. 6,552,541 B2 describes a method for correcting disturbing influences on the MR signals of a substance disposed in the measuring volume of an MR apparatus, wherein an RF excitation pulse is irradiated onto the substance, and a time-dependent MR signal generated thereby is detected and digitized in a phase-sensitive manner. A temporal behavior of the phase of the MR signal related to a predetermined reference phase of a reference signal is determined from the temporal behavior of the detected and digitized MR signal, is digitized and used to determine one or several correction or control values. In this method, almost any measuring point is used to determine the magnetic field deviation and for control. This ensures great control accuracy.
It is the underlying purpose of the invention to propose a method for generating MR images of a moving object which reliably allocates the recorded MR data with the associated motion states, thereby determining completely encoded MR data sets within an optimized period of time.